This invention relates to the trenchless installation of underground pipe, and more particularly to precise on target installation underground of a pilot pipe by microtunneling and back reaming to pull in a product pipe along the pilot pipe line.
Microtunneling and horizontal directional drilling are two conventionally used processes for the trenchless installation of new underground pipe. Of the two processes, microtunneling is used for installation of sewer pipes in view of the ability to align the pipe along a line and grade accurately. Horizontal directional drilling or “HDD” has been traditionally used for installation of utilities not requiring precise line and grade alignments such as water pipes or high-tech fiber optic cable.
In microtunneling, an entry or jacking shaft is dug, reinforced with a corrugated steel liner. A jacking or displacement machine is accurately positioned and secured in the jacking shaft with a view towards displacing a pilot pipe along a predetermined line and grade to a target shaft also lined with a corrugated steel liner. In the first phase of a typical three phase microtunneling installation, a first section of a hollow pilot tube with a steering tip at the forward end is jacked into the ground towards the target shaft displacing the soil in front of it. A light emitting diode or “LED” target is placed in the first section of pilot tube and accurate measurements of alignment are made using a theodolite securely positioned in the entity shaft. The lighted target allows for accurate for precise placement on line and grade of the pilot pipe. Steering is accomplished by rotation of the pilot pipe to rotate the steering tip. Successive sections of pilot tubes are secured to the proximal end of the pilot pipe until the target shaft is reached forming the pilot pipe. A typical pilot tube is about 4 inches in diameter.
In the second phase of the process, a reamer head with a temporary steel casing is fitted on the proximal end of the last section of pilot pipe. A central auger is positioned in the steel casing to remove soil behind the reamer. The reamer assembly is then jacked in behind the pilot pipe in sections to replace the pilot tubes and enlarge the pilot bore to desired size. A reamer as large as 20 inches in diameter may be used. The displaced pilot tubes are removed section-by-section in the target shaft. In the third phase, a product pipe is installed by jacking in behind the temporary steel casing section-by-section as described in connection with the pilot tubes. The steel casings and augers are then removed from the target shaft.
Microtunneling may also be done in two phases. Here, the first phase is identical to the three phase process with the pilot tubes jacked into the ground displacing the soil before it. In the second phase a large diameter reamer is fitted to the last pilot tube and advanced along the pilot pipe line. A pipe assembly of a product pipe with a central temporary steel casing and a central auger positioned therein is jacked in behind the reamer. Pilot tube sections are removed at the target shaft. Soil is removed by the auger and water or a drilling fluid may be injected at the face of the reamer to form a slurry that is displaced along the annulus between the product pipe and slightly larger reamer bore hold.
In horizontal directional drilling, digging entry shafts to grade below ground level are not generally required since the process is utilized for laying underground cable and the like from above the surface. The HDD process is generally used to install horizontal pipes beneath major obstacles, such as road or rivers. In this case accuracy of positioning along line and grade is not as critical as in the case of installing a new gravity-fed sewer line where deviations in line or grade are not acceptable. A directional drilling machine is positioned above ground and advances a rotating drill stem with a directional drill bit at the distal end in a direction inclined to the earth's surface to establish an initial bore hole. The rotation and advancement of the drill string are then leveled off at the required depth and then upwardly inclined back to a terminal point at the surface. The necessary deviation in line of the drill string is accomplished by a bit having a slanted face, an asymmetric drill head, eccentric fluid jets or a combination of these designs. They are typically used with an electronic locator instrument to determine position and strength of signal emitted from a transmitter in the head of the drill string of the boring system.
The drill string is in the form of a plurality of lengths of drill, generally about 15 feet in length. Each drill pipe section is provided with a male thread at one end and a female thread on the other so that the pipe may be interconnected together in sequence to provide a drill string of suitable length. The pilot bore formed by the drill string is then enlarged by a wash over pipe and a back reamer to the size required for pulling in the product pipe.
Both microtunneling and HDD methods are entirely suitable for their intended uses. The microtunneling process is extremely accurate, but relatively slow compared to the speed of installation utilizing directional drilling. The need to prepare shafts to grade level in microtunneling is avoided in the horizontal directional drilling process. However, the accuracy required for installation of new sewer pipes cannot be attained utilizing traditional horizontal directional drilling equipment and processing. This is due in large part to the inherent inability to control direction of the drill bit along accurate line and grade.
Accordingly, it is desirable to provide an improved method for accurately installing a new underground pipeline attaining the accuracy possible with microtunneling and at the speed available with directional drilling.